This invention generally relates to a method of reducing an undesired component from a signal having a desired component and an undesired component.
There are a variety of applications where separation of signal components is useful or required. In some situations, an undesired noise component must be filtered out or reduced compared to a desired sound component to achieve understandable transmissions.
One example is when an individual is speaking on a mobile telephone within an automobile. The presence of ambient noise often interferes with the ability of the person whom the individual is speaking with to hear what the individual is saying while driving. This is especially true when one attempts to use a hands-free, speaker phone within a vehicle while driving. The ability to use a speaker phone within a vehicle is desirable because it enhances safe operation so that a driver can keep both hands on the steering wheel and not be distracted from the task of driving the vehicle as much as when the driver must hold onto a cellular telephone, for example. The inability to communicate effectively, however, limits the usefulness of currently available vehicle speaker phones.
There are a variety of other applications where ambient noise is also detected by a microphone, which interferes with the projection of the desired speech component. Other applications where ease, clarity and accuracy of communication of speech is necessary include voice recognition systems, voice control systems and cabin communication systems.
With the wide variety of applications for noise reduction systems, it is not surprising that several attempts have been made to fulfill this need. One example of a noise reduction method is sometimes referred to as "spectral subtraction." This approach typically works in the frequency domain and depends upon separating speech, which typically has non-stationary statistics, from noise, which typically has stationary statistics.
While spectral subtraction methods may be useful, they are not without shortcomings or drawbacks. For example, the noise reduction provided by such methods may include musical artifacts in the reproduced speech. Additionally, there typically is a requirement for voice activity detectors in the equipment utilized to perform the method. Moreover, the spectral subtraction methods require considerable computation for Fast Fourier Transforms and may exhibit processing delays that affect the quality of the reduced speech. The large amount of computation time not only affects the quality of the reproduced speech but also can impose relatively high costs on a noise reduction system. In many applications, such as the hands-free cellular telephone within a vehicle mentioned above, the expenses associated with providing sufficient computational capability and computer memory to accomplish a spectral subtraction method typically render it not practical for such an application.
There is a need for a more efficient, more effective and more economical signal separation method that can have wide application. This invention provides such a method and avoids the shortcomings and drawbacks described above.